Endocrine Disrupting Chemicals 2012 - World Health Organization

8. Sensitive periods for endocrine disruptor
action—Windows of exposure
Figure 14. The effects of
early exposures to EDCs
may be manifested any time
in life.
Hormones and EDCs that alter hormone actions
can act at all times during life—fetal development,
infancy, early childhood, puberty, adulthood and
old age. The timing of hormone or EDC action
often determines the strength of their impact.
In the adult, the hormone or EDC has an effect
when it is present, but when the hormone or EDC
is withdrawn, the effect diminishes—much like
insulin levels rising when blood sugar is high and
then declining when blood sugar declines.
In contrast, exposure to hormones or EDCs
during development (in utero and infancy and
early childhood in humans) can have permanent
effects if the exposure occurs during the period
when a specific tissue is developing. These effects
may only become visible decades later. This is
called developmental programming. Hormones
control the normal development of tissues from
the fertilized sperm and egg to the fully developed
fetus. Since some tissues continue developing
after birth—such as the brain and reproductive
system—the sensitive period for these tissues
is extended, sometimes for decades after birth.
Developmental Exposures to EDCs Lead to Disease Throughout Life
When a tissue is developing, it is more sensitive to
the action of hormones and thus EDCs.
The mechanisms by which EDC exposure
during development can alter the development
of specific tissues, leading to increased
susceptibility to diseases later in life, are
just beginning to be understood. It is clear
that hormones play an important role in cell
differentiation, which leads to the development
of tissues and organs. Once tissues and organs
are fully developed and active, then hormones
have a different role: to control the integration
of signals between tissues and organ systems
and to maintain normal function. Early
development (when hormones are controlling
cell changes to form tissues and organs) is thus
a very sensitive time frame for EDC action. If
an EDC is present during the developmental
programming of a tissue, it could disrupt the
normal hormone levels, leading to changes in
tissue development—changes that would be
stable across the lifetime and possibly confer
sensitivity to disease later in life. These effects
Gestation Childhood Puberty
Reproductive
life
Middle
life
Later
life
Exposures
to EDCs
12 State of the Science of Endocrine Disrupting Chemicals – 2012